WASHINGTON (CNN) -- Thousands of Albanians and others who fled the Balkans for the United States in recent years have emerged as a serious organized crime problem, threatening to displace La Cosa Nostra (LCN) families as kingpins of U.S. crime, top FBI officials said Wednesday.

The Albanian criminal enterprises, operating largely in New York and other Eastern seaboard cities, represent a major challenge to federal agents because of their propensity for violence and brutality, the officials said.

"They are a hardened group, operating with reckless abandon," said Chris Swecker, the newly named FBI assistant director for the Criminal Investigative Division.

Swecker and other officials said some of the Albanians served as enforcers in the established Mafia families for several years.

When the FBI dealt a major blow to the LCN families in recent years, the Albanians began to emerge, and now are taking over turf once controlled by the traditional mob bosses.

Officials said ethnic Albanians from Kosovo, Macedonia and Montenegro are included in the clans and crews that emerged in the organized crime world.

FBI officials said Russian and many other organized crime groups operating in the United States are much more sophisticated and less violent than the Albanians.

Part of the Albanian emergence stems from the FBI's success in busting the LCN.

Operation Buttondown, the code name for the FBI's campaign to crush the Mafia, reduced the number of families operating in the United States from 24 to only nine, the FBI officials said.

More than 100 leading members of LCN and more than 600 associates were arrested in the crackdown on organized crime.

Although no terrorism connection has been discovered, the FBI says it is closely watching to see whether the militant Muslims in the emerging organized crime world demonstrate ties to organizations suspected of involvement in terrorist financing.